1. Field of the Invention
The present invention relates to a high voltage charging circuit and, more particularly, to a high voltage charging circuit for charging a high voltage capacitor.
2. Description of the Related Art
For cameras, when a user needs to take a picture in insufficient light, the camera utilizes a flash to provide extra light to obtain better exposure effects. However, a working voltage (such as 300 volts) of the flash is much higher than a voltage (such as 5 volts) of a direct current used in the camera; therefore, a high voltage charging circuit is utilized to raise the voltage of the direct current by using a transformer with a high transformation ratio to charge a capacitor. When a voltage on the capacitor reaches the working voltage of the flash, the capacitor can be used to provide current to the flash.
As shown in FIG. 1, a conventional high voltage charging circuit 10, also known as a ring chock converter (RCC), comprises: a direct current power source 12 (for example, a 5 volt supply), three resistors 14, 16, and 30, a power transistor 18, a transformer 20, a diode 22, a high voltage capacitor 24 (for example, a 300 volt capacitor), a Zener diode 26 (with, for example, a breakdown voltage of 300 volts), a capacitor 28, and an idle control circuit 32. The transformer 20 has a primary side winding N1, a secondary side winding N2, and an auxiliary winding N3. The primary side winding N1 and the auxiliary winding N3 induct with the secondary side winding N2; the primary side winding N1 and the secondary side winding N2 are oppositely polarized, and the number of coils on the secondary side winding N2 is N times the number of coils on the primary side winding N1 (e.g. 60 times).
When the direct current power source 12 provides current to the transformer 20, the resistor 14 and the primary side winding N1 are inductive with each other, the power transistor 18 is in a saturated state, and the auxiliary winding N3 and the resistor 14 are inductive with each other. A current passes through the primary side winding N1, which is a magnetically induced current that stores its energy in the transformer 20 instead of charging the high voltage capacitor 24.
When the current passing through the resistor 14 increases, the power transistor 18 moves to an active state, which reduces the current passing through the primary side winding N1, and so the polarities of the primary side winding N1 and the auxiliary winding N3 are reversed. Then, the power transistor 18 transitions into a cut-off state, and the secondary side winding N2 and the diode 22 become inductive with each other to perform a charging process on the high voltage capacitor 24 with a charging current. When the secondary side winding N2 transfers the energy stored in the transformer 20 to the high voltage capacitor 24, the primary side winding N1 goes back to its original state, and becomes inductive with the resistor again.
When the voltage on the high voltage capacitor 24 reaches a predetermined value (for example, 300 volts), this voltage will cause the Zener diode 26 to breakdown and short, which activates the idle control circuit 32 to stop the transformer 20 from charging the high voltage capacitor 24.
FIG. 2 is a graph of a charging current with time. As shown in FIG. 2, only when the charging current drops back to zero, the transformer 20 starts to provide the charging current to the high voltage capacitor 24 for charging.
Accordingly, the conventional high voltage charging circuit 10 has the following disadvantages:
(1) The power transistor 18 is a BJT, which needs a base-polar driving current, and has a saturation voltage VCE of about 300 mV. Therefore, the power transistor 18 itself wastes a lot of energy and thus leads to a poor charging efficiency of the high voltage charging circuit 10.
(2) Since the transformer 20 needs the auxiliary winding N3, and as the switching frequency of the power transistor 18 is usually 100 kHz, the size of the transformer 20 is not easy to minimize.
(3) Since the conventional high voltage charging circuit 10 does not operate under a continuous conduction mode, it has a low efficiency.
Therefore, it is desirable to provide a high voltage charging circuit for charging a high voltage capacitor to mitigate and/or obviate the aforementioned problems.